First man with a barrier (4)
I believe the answer is:
adam
'first man' is the definition.
(first man in the Bible)
'with a barrier' is the wordplay.
'with' says to put letters next to each other.
'barrier' becomes 'dam' (dam is a kind of barrier).
'a'+'dam' is 'ADAM'.
(Other definitions for adam that I've seen before include "The first known ancestor" , "who transformed Palladian architecture?" , "Male" , "Eden inhabitant" , "Scots. architect brothers" .)